A level detector or sensing switch usable in a brake-fluid reservoir is described in U.S. Pat. No. 3,934,103. This arrangement has a float that is in the reservoir and that can pivot about a horizontal axis. A bar magnet extending perpendicularly across the axis has an end juxtaposed through a nonmagnetic wall of the reservoir with a ferromagnetic ball displaceable between a lower open-circuit position out of engagement with a pair of contacts and an upper closed-circuit position engaging a pair of contacts. As the liquid level inside the reservoir drops the magnet tips and slowly moves this ball upwardly until it engages the contacts in the upper position and closes a circuit indicating that the fluid in the reservoir has dropped below a safe level.
This system has shown itself in practice to have several disadvantages. Mainly when subjected to a severe jolt the ball can frequently be bounced upwardly into the closed-circuit position so that the arrangement gives a false warning. What is more when it is properly displaced into the upper position and held therein by the magnet, a severe jolt will allow it to drop downwardly into the open-circuit position. In this lower position the magnetic force is not sufficient to raise it back up, so that although the float is down, indicating that the fluid level is too low, no warning will be given. Finally it is noted that the ball itself only lightly and normally intermittently engages the contacts, so that at best the warning light will flicker.